One of the better environmentally resistant coatings for aluminum is prepared by chromic acid anodizing, and therefore many structural aluminum parts are anodized with chromic acid or a dichromate. It is known to determine the corrosion resistance of a chromic ion anodized aluminum by use of a salt spray continuous exposure test that requires fourteen days. During the testing all production anodized parts have gone on to the next steps in production under an uncertainty as to quality. Should the test show a defective anodize, the production part must be removed. Therefore, it would be desireable to have a quick method of testing the anodized coat on aluminum structures. In U.S. Pat. No. 2,902,348, a drop of an acidic ferric chloride-potassium ferricyanide solution when placed onto a chromate coated aluminum would change color within a minute, provided the coating was defective.
It was discovered that a reliable quantitative test of the corrosion resistance of a chromic acid anodized coating for aluminum could be completed in about 30 minutes.